Question for the Motor Heads

[Sutts]

Originally Posted by ARM505

Again, I must ask/clarify:

In the following situation - constant everything (ie altitude, throttle setting, mixture setting, airspeed etc), a change in RPM via the prop control lever will result in (and I have tried both the A2A model and CLOD in similar density alts/airspeeds, for what it's worth):

a) a slight decrease in indicated boost (ie the A2A model Spit)

b) an increase in indicated boost (ie the CLOD model)

One is correct, the other not. So....which is it?

You haven't said whether you're increasing or decreasing RPM with the prop control.

Increased RPM.....lower boost
Reduced RPM....higher boost

[ATAG_Dutch]

Originally Posted by Sutts

You haven't said whether you're increasing or decreasing RPM with the prop control.

Increased RPM.....lower boost
Reduced RPM....higher boost

He must be reducing rpm as the CoD model would show an increase.

[Sutts]

Originally Posted by Dutch_851

He must be reducing rpm as the CoD model would show an increase.

Sounds like CoD is modelled correctly then, thanks.

[ARM505]

Originally Posted by Sutts

You haven't said whether you're increasing or decreasing RPM with the prop control.

I'll just quote my original post again quickly since it somehow didn't get through....I'll use the exact same wording since I just can't put it any other way :

Originally Posted by ARM505

...a change in RPM via the prop control lever will result in...

And here's the question, since I am indeed asking a question:

Originally Posted by ARM505

One is correct, the other not. So....which is it?

ie which is correct, the A2A model, or the CLOD model, since they show the exact opposite result?

[Sutts]

Originally Posted by ARM505

I'll just quote my original post again quickly since it somehow didn't get through....I'll use the exact same wording since I just can't put it any other way :



And here's the question, since I am indeed asking a question:



ie which is correct, the A2A model, or the CLOD model, since they show the exact opposite result?

Still don't get it I'm afraid....you just say a change. A change UP or a change DOWN in RPM??

[winny]

Originally Posted by Blakduk

Winny- I've been googling Alex Henshaw this evening. I didn't realise he was such a celebrity!

I thought you didn't really realise who he actually was

He's a bit of a legend Spitfire wise, you were lucky to meet him.


Anyway I don't want to interrupt the thread too much.

Now what is it again? More throttle to go faster, less to go slower, ignore gagues - repeat till engine explodes?

[Blakduk]

Back on topic, a slight reframe of the query:
- throttle steady, maintain altitude (and therefore air pressure is same)
- change pitch to coarse, increasing load on engine, dropping prop & engine RPM
- drop in engine RPM drops RPM of supercharger impeller, decreasing amount of air/fuel forced into inlet manifold
- drop in engine RPM decreases amount of air/fuel being sucked into cylinders.

Question- is the decrease in air/fuel negative pressure at inlet to cylinders more or less than the decrease in the positive pressure of air being forced into the inlet manifold by the supercharger?
If the decrease at the cylinder end of the manifold is more, the boost gauge will read higher as the boost gauge measures increased air pressure in the inlet manifold as there is a backup of air/fuel.

[ARM505]

I'll ask my question another way.

I'm comparing two different models (FSX vs CLOD), both of a Spit MkII with a constant speed propeller:

1) The A2A simulations Spit Mk II running in FSX. In this model, whilst flying at a constant altitude, constant airspeed, constant throttle setting, the indicated boost will DECREASE when you move the prop control lever to DECREASE RPM - the A2A docs mention that since the supercharger is now being driven at a lower RPM, the indicated boost will decrease. The decrease is however very slight.

http://shockwaveproductions.com/

2) The CLOD model, which (under the same conditions), will show an INCREASE in indicated boost when the RPM is commanded to a lower value via the propeller control lever, ie the opposite of the A2A model in FSX.

Since they display the opposite behaviour, one is therefore correct, and the other incorrect. My question is thus which is most true to life. The A2A Spit is widely recognised as a very well modelled aircraft in FSX terms. I have in the past accepted it's behaviour (with the exception of the rapid wear issues) as well modelled. I would like to find out if this is true in this respect.

[Babi]

Originally Posted by Sutts

If boost rises when rpm falls (at a fixed throttle setting) and boost falls as rpm rises then the sim is correct.

As mentioned earlier, an engine is just like a big suction pump, sucking air fuel mixture out of the inlet manifold and thereby reducing the pressure in that manifold (which shows on the boost gauge).

The higher the RPM (at a fixed throttle setting), the faster the air is pumped out of the manifold and the lower the manifold pressure becomes. On the other hand, when RPMs are reduced (at a fixed throttle setting), less air is being sucked out and the pressure rises.

When the engine isn't running, the suction pump stops and the pressure in the manifold returns to the atmospheric pressure at the current altitude being flown.

On a non-supercharged / non-turbo charged engine, the highest boost pressure available is the atmospheric pressure at the current altitude.
On a supercharged / turbo charged engine, the highest boost pressure available is the max pressure provided by the compressor at the current altitude.

That's my understanding anyway.

It's true that pressure drops with increased RPM in a normal (aspirated) engine because of "suction". But you completely miss the other important factor: the supercharger is not some magical box that gives its nominal boost output regardless of what the engine is doing. Its mechanically driven by it, and the faster the engine RPMs, the faster the supercharger spins and the more boost is able to deliver. The pressure ratio the supercharger is able to achieve does not vary linearly with its RPM, and depends on the specific type of compressor installed. So you see? the pressure tends to go down because of increased RPMs, but the supercharger tends to deliever more boost because of this RPM increase. What is the predominant effect? difficult to say, depends on supercharger and intake design, and also if the engine is running at low or high RPM.

Read my post on top of page 5.

In the end we can't say anything about the accuracy of the model, until we have specific engine test data or supercharger characteristics.

[MadBlaster]

Originally Posted by SYN_Bliss

But the exhaust side of the equation has nothing to do with anything in the intake manifold. The exhaust valves only open during the exhaust stroke and only push exhaust out the exhaust manifold/header.

The intake valves start to open up as the piston is at TDC (top dead center) and as the piston goes down it fills the cylinder with air and fuel. This step is the only suction coming from the intake manifold where the supercharger is supplying forced induction. Next that same cylinder compresses and at a certain point in the ignition cycle during this compression stroke, the spark plug fires and creates an explosion. This causes the piston to go down again (cylinder is now full of exhaust gas) and on it's trip up the exhaust valve is open to let it leave the cylinder head through the exhaust manifold/header.

There's ongoing exhaust back pressure in almost virtually every combustion engine. If you didn't have back pressure, you would easily burn valves amongst other problems. To give you an idea about the Merlin (from memory) the exhaust manifold design was changed at a backwards angle because of the sheer amount of thrust it created. The exhaust gases are flowing either 700 to 1400mph out of the exhaust manifold (don't remember), and it didn't take long before someone realized that if they angled the manifold a certain way, that it would actually increase top speed, help with propulsion.



Not necessarily. It depends on your starting RPM, the output of the supercharger, the engine etc etc. But generally if you are spinning 75% of max RPM, and significantly increase load to decrease RPM on a s/c'd engine boost will increase. There will be a point where this won't happen anymore. An engine creates vacuum. So at a certain point you will not have any positive pressure anymore because the blower will not keep up, unless****, while more load is added you are opening the throttle. Again, this all depends on so many variables and specifics of the engine/sc combo that I don't think anyone can give you a good answer. But if you remember that increased load and decreased RPMs will give you boost (in a boosted app) but only to a certain point.

I understand what your saying and agree, but I think about it differently. Consider that this is a multi-piston situation, air being a fluid and the rpms being so high. So at any moment in a cycle, one piston has intake valve opening and sucking air (creating negative force flow on the downstroke), another piston has exhaust valve opening pushing spent gas (creating positive force flow on the upstroke) and the other cylinders have their valves closed because they are either compressing fuel/air or driving the shaft from ignition. So if the load increases and rpm drops, the engine cylinders are moving less air through the "system" (negative and positive flows from the system are reduced). In my mind, this creates the air "backstop" that allows the supercharger to build up pressure in the intake manifold, not just the longer time that the intake valves are closed because that's only half the system that is moving air through the engine.